Boundary layer control of flow separation and heat exchange

ABSTRACT

Boundary layer control for delay or prevention of flow separation and/or increase in rate of heat exchange between a surface and a fluid by an arrangement of surface elements which may take the form of either crests or discreet concave depressions in the surface, having effective depths or dimensions of less that of the adjacent boundary layer thickness, to cause the formation of vortices with succeeded surface elements being positioned to cause vortex amplification, for effective boundary layer mixing with less drag, weight penalty, noise, and energy loss than that of conventional vane-type generators.



1. Apparatus for the displacement or delay of the inception of flow separation of a fluid flowing over a curved surface, comprising means defining a plurality of elements formed on said surface and defining with said surface concave vortex generating regions, with some of said elements being arranged in arrays defining a plurality of rows oriented generally transversely to the direction of flow with said rows being spaced in downstream relation to other of said rows a distance of between five and twenty times the boundary layer thickness to effect vortex amplification, and said elements each having an effective height defining said concave regions which is less than the thickness of the adjacent boundary layer.
 2. The apparatus of claim 1 in which at least some of said elements form crests on said surface oriented generally transversely to the direction of flow and defining on their downstream side said concave regions.
 3. The apparatus of claim 2 in which said crests have a height equal to approximately one-half the boundary layer thickness.
 4. The apparatus of claim 1 in which at least some of said elements are formed by means in said surface defining troughs having their axes oriented generally transversely to the direction of fluid flow.
 5. The apparatus of claim 1 in which the elements in each said row are positioned at an angle to each other and to said direction of flow in alternating zigzag fashion to form an array of said elements which generate adjacent vortices of opposite signs.
 6. The apparatus of claim 5 in which the elements of a downstream row are placed so as to intercept and amplify vortices of the same sign as generated by those of an upstream row.
 7. The apparatus of claim 1 in which said arrays alternate between crests and troughs in the direction of fluid flow.
 8. The apparatus of claim 1 in which said elements are arranged in a series of V-shaped arrays on said surface in which the apex of said arrays are oriented toward the direction of fluid flow and in which streamlines in the flow bisect the included angles of said arrays.
 9. The apparatus of claim 8 in which the half angle of said bisected angle is an acute angle and is less than the angle the sine of which is the reciprocal of the local Mach number at the edge of the boundary layer.
 10. The apparatus of claim 1 further including an aircraft wing, a Fowler-type flap movable on said wing between a retracted position in which a surface portion of said flap is recessed into said wing and an extended position in which said flap portion is exposed and defines with said wing a transverse slot, and in which said elements are formed on said flap surface portion.
 11. Vortex inducing apparatus in combination with a fluid control or flow directing surface for generating vortices by utilizing the energy represented by the boundary layer vorticity upstream of the apparatus so as to cause mixing of mainstream fluid with boundary layEr fluid downstream of the apparatus, comprising at least one elongated flow diverting element arranged in superimposed relation with respect to said surface and having a height proportioned to extend partially into the boundary layer and having a length which is substantially greater than the upstream boundary layer thickness extending in a direction generally transversely but inclined at an angle other than 90* to said mainstream flow and being formed with a downstream surface defining a correspondingly transversely oriented concavity with said flow control surface in which the direction of flow of some of the streamlines closely adjacent said downstream surface is deflected to a greater extent than the streamlines in the boundary layer more remote from said downstream surface to generate rotation of said streamlines resulting in a trailing vortex substantially within the dimensions of the original upstream boundary layer.
 12. The apparatus of claim 11 in which said element extends transversely along a generally straight line.
 13. The apparatus of claim 11 further comprising a plurality of said elements arranged relatively transversely one to the other for forming a corresponding plurality of individual vortices.
 14. The apparatus of claim 13 in which said elements are formed separately from said flow control surface with a generally cylindrical upper surface and a generally flat bottom surface for attachment to said flow control surface.
 15. The apparatus of claim 13 in which said elements are of uniform cross-sectional dimensions throughout their length.
 16. The apparatus of claim 15 in which said elements extend transversely along generally straight lines.
 17. The apparatus of claim 13 in which said plurality of elements are formed in adjacent pairs inclined oppositely from each other in relation to said mainstream flow and joining at a common apex oriented toward the direction of flow for forming a pair of adjacent vortices of opposite sign.
 18. The apparatus of claim 17 adapted for use in a supersonic flow environment in which said pairs of elements define therebetween an included angle less than the Mach angle of the mainstream flow upstream of said elements.
 19. The apparatus of claim 13 in which said elements are arranged with respect to said mainstream flow with alternating inclinations thereto in a generally zigzag manner to form a corresponding array of adjacent vortices of alternating signs.
 20. Apparatus for delaying flow separation of air forming over a curved flow control surface by creating vortices from a region within the boundary layer by utilizing the boundary layer vorticity to mix mainstream flow of air with boundary layer air and impart energy to said boundary layer air, comprising a transversely oriented series of essentially rigid elongated flow diverting elements mounted in superimposed relation to said surface, each of said elements having a height proportioned to extend only partially into the boundary layer and each having a length which is substantially greater than the boundary layer thickness extending on said surface generally transversely to said mainstream flow but inclined at an angle to said flow other than 90* , and each being formed with a downstream surface which defines a correspondingly oriented concavity with said flow control surface, said elements being positioned on said flow control surface upstream of the region for which flow separation is desired to be delayed and having a width in the direction of flow which is substantially less than said transverse length and being of generally uniform cross-sectional dimension throughout the length thereof for effecting a reversal of the direction of flow of some of the boundary layer streamlines projected on a plane to the axis of said element closely adjacent said concavity and further affecting a deflection of streamlines within the boundary layer which are more remote from said concavity so as to generate a series of trailing vortices, one for each of said elements, said vortices being generated substantially within the dimension of the original boundary layer.
 21. The method of delaying or preventing flow separation in fluid flow over a curved flow control surface comprising the steps of placing a surface element entirely in the boundary layer to form a streamwise vortex at a discrete upstream location within the boundary layer and placing additional said surface elements at downstream locations to intercept and amplify such vortex at such downstream locations on said surface within the boundary layer.
 22. The method of mixing boundary layer fluid with a mainstream of fluid moving along a fluid controlling surface, comprising the steps of forming a transverse discrete condition on said surface entirely within the boundary layer and along a spanwise extent of said surface which extent is substantially greater than the boundary layer thickness and at an angle of other than 90* to the direction of mainstream flow creating a corresponding extended spanwise concavity which is also inclined to the direction of flow of said mainstream at an angle other than 90* , said concavity causing a differential deflection of the streamlines within the boundary layer along the length thereof so that the energy represented by the boundary layer vorticity is diverted into the formation and enhancement of a vortex substantially within the dimensions of the original boundary layer, said vortex trailing off downstream from said transverse concavity.
 23. The method of mixing boundary layer air with a mainstream of air moving over an air controlling surface, comprising the steps of placing an elongated element on said surface entirely within the boundary layer and orienting said element at an angle to the mainstream flow other than 90* for deflecting the streamlines of the boundary layer air along a corresponding extended transverse region at the lee side of said element causing a concave deflection of some of the streamlines within the boundary layer spaced from said element and a flow reversal of others of the boundary layer streamlines projected on a plane normal to the axis of said element closer to said lee side resulting in the formation of a vortex substantially within the dimensions of the original boundary layer. 